The objective of the proposed research is to study the relationship of cardenolide C(17)-"side group" geometry to biological activity. Collaborative research of the applicants has revealed that cardenolide activity is very responsive to subtle configurational and conformational changes in the side group. The proposed research seeks to delineate this relationship. The proposed research will include: (a) Synthesis of new cardenolide analogues, designed on the basis of previous work of the applicants to probe the geometric requirements of cardiac glycoside receptors. (b) Screening of all the new analogues for cat heart Na ion, K ion-ATPase inhibition properties and for inotropic activity with guinea pig atria. For reasons of cost and the large number of animals required, only selected analogues will be evaluated for inotropic activity with cat atria. (c) For analogues with particularly interesting biological activities, for AY22, 241, and for one cardiac glycoside, x-ray crystallographic and subsequent conformational energy studies will be made. (d) Additional biological studies (e.g., Ca ions binding) will be done on a few compounds, selected on the basis of the studies above.